Isoreticular Two-Dimensional Covalent Organic Frameworks Synthesized by On-Surface Condensation of Diboronic Acids

Abstract

On-surface self-condensation of 1,4-benzenediboronic acid was previously shown to yield extended surface-supported, long-range-ordered two-dimensional covalent organic frameworks (2D COFs). The most important prerequisite for obtaining high structural quality is that the polycondensation (dehydration) reaction is carried out under slightly reversible reaction conditions, i.e., in the presence of water. Only then can the subtle balance between kinetic and thermodynamic control of the polycondensation be favorably influenced, and defects that are unavoidable during growth can be corrected. In the present study we extend the previously developed straightforward preparation protocol to a variety of para-diboronic acid building blocks with the aim to tune lattice parameters and pore sizes of 2D COFs. Scanning tunneling microscopy is employed for structural characterization of the covalent networks and of noncovalently self-assembled structures that form on the surface prior to the thermally activated polycondensation reaction.

Principles of molecular assemblies leading to molecular nanostructures

“…The ability of a surface to act as an additional ligand for porphyrins was reported earlier by Elemans and co-workers [65] for alkylated Mn-porphyrin monolayers adsorbed at the tetradecane/Au(111) surface. Recent years have witnessed a surge in reports describing surface-supported synthesis and STM of two-dimensional covalent networks [66][67][68][69][70]. Selective intermolecular interactions have been widely exploited to couple non-covalently assembled molecular components in a more 'permanent' covalent fashion.…”

Self-templated chemically stable hollow spherical covalent organic framework

“…These materials showcase promising applications in the field of gas storage [2][3][4][5] , opto-electronics 6 , catalysis [7][8][9] and sensing 10 because of their crystalline and periodic structure with high surface area and thermal stability. COFs are usually synthesized as microcrystalline powders and their long-range growth is limited in the nano/micro domain due to the internal defects [11][12][13] . COF crystallites have been reported to have adopted various shapes such as belts 6 , fibres 14 , sheets 15 , cubes 16 , rectangular prisms 17 , spheres and platelets 18 .…”

Recent Progress in the Fabrication of Low Dimensional Nanostructures via Surface-Assisted Transforming and Coupling

“…Taking advantage of different monomers with alternative length of the straight organic backbones in the diboronic acids, not only can the size of pore be changed, but also the figure of the planar boroxine rings can be transformed from one to another, such as the implemented scheme showed in Figure 6(d) that the condensation of boronic acids occurred between 1,4-benzenediboronic acid (BDBA) and 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) simultaneously generating a five-membered BO 2 C 2 ring [45], being different with the formed planar boroxine rings (B 3 O 3 ). The lattice parameter and pore size of the 2D SCOF are obviously different leading to the discrepancy between two types of monomers, biphenyldiboronic acid and terphenyldiboronic acid, used for polymerization reaction (Figure 6(b)) [46]. Therefore, ignoring the long-range ordered characteristics of the polymers, the topological diversity of nanostructure can be used to tune the electronic property of SCOF.…”

“…In particular, the condensation at liquid/solid interface depends on the reversibility or gas/solid interface in the closed systems while the water is being supplied by some novel path; for example, blue vitriol (CuSO 4 ⋅5H 2 O) reversibly releases part of its crystal water already around 100 ∘ C making it possible for removing the defects in long ranged ordered COF [57,58]. As shown in Figure 6(c), the water molecules released from CuSO 4 ⋅5H 2 O during the heating process can act as an equilibriummanipulating agent to push the dehydration reaction backward and thus promote defect remedy process to form a highly ordered network, and the water released from CuSO 4 ⋅5H 2 O would be reabsorbed to CuSO 4 again during the cooling process and prevent the potential decomposition of SCOFs [46]. In other words, controlling the temperature of the reaction straightly related to the amount of H 2 O supplied by the temperature-dependent and reversible dehydration of CuSO 4 ⋅5H 2 O.…”

“…In order to convert a noncovalent self-assembled monolayer into a surface-supported 2D COF, several different reacting steps, such as breaking up of hydrogen-bonded aggregates, surface diffusion, reorientation, and possibly desorption, need to be overcome. Thermal activation plays an important role not only in initiating the polycondensation but also in breaking up the preceding noncovalent arrangement, that is, the self-assembled monolayer [46]; meanwhile it can properly increase the surface mobility which is limited at room temperature leading to low degree of order in these kinetically trapped structures [59]. Besides the advantage of reversibility of B-O bond, the monomers dissolved eventually in solution can deposit on solid substrate at the same speed, which can prominently improve the quality of SCOF.…”